This invention relates to scanning radio receivers, and more particularly to scanning radio receivers with frequency synthesizers.
Early scanning receivers used crystals for tuning, and the number of channels available for reception was limited by the number of crystals in a given receiver. A receiver typically contained either eight or sixteen crystals, and different crystals had to be installed to enable reception of different frequencies, including the frequencies of other desired transmissions within range of the receiver as well as, in the case or relocation or portable operation, the assigned frequencies in a different geographical region.
Modern scanning receivers generate local oscillator frequencies with a frequency synthesizer controlled by frequency codes stored in memory. Typically, the memory still contains a small number of frequency codes, but it can be programmed for operation on different frequencies. The channel capacity of scanning receivers has heretofore been limited not only because of the size, cost and complexity of memory circuitry and associated addressing circuitry, but also because of technical limitations on scanning speed. A finite amount of time is required to lock on to each frequency in the active scanning sequence, and then to detect activity on the current channel in order to determine whether or not to continue scanning.
In some applications it would be desirable to scan more than sixteen channels, and to periodically change the frequencies in the scanning sequence, without the time-consuming and inconvenient task of reprogramming memory. For example, commercial truckers commonly have reason to monitor police-band radio activity in the various geographical regions, e.g., states, in which they operate. Many states have more than sixteen channels allocated by the FCC for police use, and the frequency allocations vary from state to state. As a result of these and other factors, conventional scanning radio receivers cannot satisfactorily provide regional scanning.